Shelly sells a smarthome sensor called Door/Window2. It works well, use wifi, is very configurable and can be used locally, without cloud. Unfortunately they’ve chosen a weird, expensive battery option, CR123A. It can cost around 70 NOK (~10 USD), and does not exist as a rechargable option. And you need two of them! So what do you do if you only have one left?

Use a single, abundant 18650 instead!

“If you can’t measure it, you can’t improve it.”

Reducing energy usage is hard if you can’t see it. Here’s a cheap way to graph home or office energy usage.

By reading data from the energy meter, one can get super accurate, real-time usage data.

I’ve used:

• A smart home controller, Home Assistant
• A graphing system, Grafana
• A micro controller, esp8266, ~$5
• An LDR sensor, $1

You need some familiarity with all of the above to set this up.